CA1149918A - Frequency modulated railroad track circuit - Google Patents

Frequency modulated railroad track circuit

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Publication number
CA1149918A
CA1149918A CA000342152A CA342152A CA1149918A CA 1149918 A CA1149918 A CA 1149918A CA 000342152 A CA000342152 A CA 000342152A CA 342152 A CA342152 A CA 342152A CA 1149918 A CA1149918 A CA 1149918A
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Canada
Prior art keywords
signal
track
rails
frequency
coupled
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
Application number
CA000342152A
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French (fr)
Inventor
Mario Poggio
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Wabco Westinghouse Italiana Segnali SpA Cia
Original Assignee
Wabco Westinghouse Italiana Segnali SpA Cia
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Application filed by Wabco Westinghouse Italiana Segnali SpA Cia filed Critical Wabco Westinghouse Italiana Segnali SpA Cia
Priority to CA000342152A priority Critical patent/CA1149918A/en
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Publication of CA1149918A publication Critical patent/CA1149918A/en
Expired legal-status Critical Current

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Abstract

(Case No. 7033) ABSTRACT OF THE DISCLOSURE
A frequency modulated track circuit arrangement in which a transmitter supplies an FSK signal to the rails, with side frequencies deviating by equal amounts from a central carrier frequency and alternating at a preselected modulation rate.
The receiver apparatus includes first and second verification circuit networks coupled to the rails through a band pass filter network which passes only the correct side band fre-quencies based on the central frequency of the transmitter.
The first verification network demodulates only the correct side frequencies and decodes the resulting signal to produce an output only if the modulation rate is correct. The second verification network comprises an amplifier with a predeter-mined threshold level based on the known attenuation of the transmitted signal by the track circuit and produces an output only if the track signal equals or exceeds this threshold level. The two network outputs, only if both are present, activate an AND gate which supplies an output signal to a delay circuit element which energizes the associated track relay only if the duration of the verification outputs exceeds the preset delay period which eliminates very short noise pulses possibly having true characteristics.

Description

(Case No. 7033) ~1~99~8 FREQUE~CY MODULATED RAILROAD TE~ACK CIRCUIT

BACKGROIJND OF THE INVENTION
My invention pertains to a frequency modulated railroad track circuit. More particularly, the invention pertains to an audio frequency track circuit in which an audio frequency signal is modulated by frequency shift keying to provide a modulated track signal whose accurate reception may be veri-fied at the receiver to increase the safety of the track circuit operation.
All railroad track circuits, and particularly audio frequency overlay circuits, are subject to extraneous noise signals induced from sources other than and external to the track circuit source. This noise may interfere with the operation of the track circuit or even cause the registration of incorrect indications as to track occupancy conditions.
Such interference i8 particularly prevalent in electrified railroads where propulsion current flows in the rails. ~u~
F~e7~e hC~/
e~ret~ Y~ e~ modulation (FM) techniques to modulate the transmitted track circuit signals in order to provide greater distinctiveness from the noise signals for the track circuit receiver have been used, frequently in the form of Frequency Shift Keying (FSK). However, frequency modulation itself is not the complete answer to improved reliability and safety.
The conventional FM receiver apparatus used may not detect the absence of one of the side band frequencies due to an extremely low level of the received track signal resulting from an apparatus fault or a broken rail. In addition, the ~149~i8 external noise may match one of the side band frequencies, although at a lower level. Therefore, a track circuit receiver arrangement is desirable which will check that the signal received from the rails has the proper characteristics and ``
conditions. For example, one of three very desirable checks is to determine that both side band frequencies are present and alternating at the initial modulation frequency or code rate. Further, the received signal should have the proper level determined in accordance with predetermined track section attenuation characteristics and, even if it has these proper characteristics, should also have a time duration of at least a predetermined minimum length which will eliminate very short induced pulses which may have equivalent charac-teristics.
Accordingly, an object of the invention is a frequency modulated railroad track circuit arrangement which verifies the reception of a proper frequency modulated track signal of predetermined sufficient signal level at the receiver end of the section prlor to enabling the registry of an unoccupied track section indication.
Another object of the invention is receiver apparatus, for a frequency modulated audio frequency railroad track circuit, which includes verification means to aseure that the received track signal correctly contains both side band frequencies of the central carrier signal and that the received signal is of at least a preselected minimum level and duration.

., -~149918 Still another object of the invention is a frequency shift keyed overlay track circuit which includes verification networks at the receiver location, to check the reception of a track signal having identical frequency characteristics to that transmitted and a preselected level in accordance with the known attenuation characteristics of the track section prior to enabling the registry of an unoccupied section indi-cation.
Also an object of the invention is a receiver arrangement, for a frequency modulated track circuit, which includes a first verification network, responsive only to a received track signal containing both side frequencies of a frequency shift keyed carrier signal and which occur at the correct modulation rate, and a second verification network, respon-sive only to a received track signal with at least a predeter-mined minimum level, and which registers an unoccupied track section indication only when both networks re~pond to the received signal for a predetermined minimum time duration.
Yet another object of the invention is an alternating current railroad track circuit arrangement in whieh a fre-quency shift keyed carrier signal i~ transmitted through the rails from one end of the railroad traek seetion to reeeiver apparatus at the other end whieh ineludes a first channel verifying that both side band frequencies are received at the correct modulation rate, a second channel verifying that at least a minimum signal level of modulated carrier is received, and logic registry means to indicate an unoccupied track ~1499i8 section condition only when the existence of the corresponding distinctive conditions are verified by both channels over a predetermined time period.
Other objects, features, and advantages of the invention will become apparent from the following specification and appended claims when taken in connection with the accompanying drawing.

Sl~MMARY OF THE INVENTIOl~ .
According to the invention, track circuit transmitter apparatus, connected to the rails at one end of a railroad track section, generates and transmits through the rails an alternating current signal having a basic or central carrier frequency which may be in the audio range. This central fre-guency i8 modulated by the FSK method so that the actual track signal has two ~ide band frequencies which deviate from the central carrier in opposite directions by the same amount and which alternate at the selected modulation rate. At the other end of the section, track circuit receiver apparatus is connected to the rails to receive the track signal and energize a track relay only when the received signal has frequency and modulation characteristics corresponding to the transmitted signal, has a signal level above a threshold predetermined in accordance with the transmitted signal level and track section attenuation characteristics, and has a duration in excess o a predetermined ~afe delay period.
In the specific showing, the track circuit transmitter comprises a relatively conventional combination of elements _ 4 --to generate an FSK signal which is filtered and amplified for transmission through the track section rails. It provides a frequency modulated signal with two side fre-quencies spaced about a central carrier frequency and deviating an equal amount on both sides at a selected modulation rate. The receiver apparatus includes two veri-fication channels or circuit networks, each verifying the presence, in the received signal, of predetermined charac-teristics of the transmitted signal. Each of these networks is fed from the rails through a common input network including two filters and an amplifier coupled in series. The filters are both of the band-pass type, the first passing a frequency band centered about the modulated central frequency, and the second passing the two side freguencies of the FSK signal.
The filtered signal is amplified to a sufficient operating level for verification. The fir~t verification network receives the filtered signal with the side band frequencies and demodulates thi~ signal to determine the presence of both frequencies at the proper spacing from the central carrier frequency. The demodulated signal is then decoded to as~ure that the modulation rate of the received signal is the same as that of the transmitted signal. If these condi-tions are met, the first network supplie~ an output signal to one input of a logic AND element. The second verification channel or network receives the filtered and amplified signal from the input network and, by a conventional linear amplifier, determines whether the received signal has at leat a minimum ,:

~1499~8 level, that is, equals or exceeds a preset threshold level of this amplifier. The selected threshold level is determined by subtracting the predetermined attenuation caused by the characteristics of the track rail circuit from the known signal level transmitted from the other end of the section.
If this threshold is at least equaled, an output from the linear amplifier is supplied to the second input of the logic AND element. If both verification or concurrence signals are present, the output of the AND gate is supplied through a delay circuit to energize the track relay. This delay element requires that the output of the AND gate be of at least a pre-determined duration to eliminate very short, extraneous noise signals induced into the track circuit which might meet the other verification tests. The track relay is energized then only if the received signal is verified as having the proper characteristics, that i8, an unoccupied track ~ection indica-tion i8 registered only by the reception of a correct signal.
DESCRIPTION OF THE PREFER~ED EMBODIMENT
Prior to defining the invention in the appended claims, I shall describe a preferred embodiment of the invention as illu~trated in the accompanying drawing which shows schemati-cally, and principally by block diagram, the transmitter and receiver apparatus for a frequency modulated railroad track circuit embodying the invention.
Referring now to the drawing, across the top are conven-tionally shown the two rails 1 and 2 of a section T of a stretch of railroad track. Connected to the rails at the ~9918 right end of the section is a track circuit transmitter, diagrammatically shown below the horizontal dashed line separating the upper and lower parts of the drawing. Con-nected to the rails at the left end of the section is the track circuit receiver which is similarly illustrated above the dashed line. The rails of the section T may or may not be insulated from the adjoining rails of the stretch by insulated joints although none are here shown. The track circuit of the invention is preferably used in an overlay fashion without insulated joints, e. g., for a highway crossing warning installation. However, as is well known in such track circuits, the receiver apparatus is tuned to respond only to the signal from the corresponding track circuit transmitter so that no interference with other track clrcuits occurs, for example, such as used for traffic control signaling.
The transmitter include9 a modulation signal or code generator 6 which produces the modulation signal having a frequency Fm which may be on the order of 7.5 to 20.0 Hz.
Any known arrangement, preferably using solid-state elements, may be used to produce this code or modulation frequency signal. A signal generator and modulator unit 8 produces a signal, having a central carrier frequency Fc, which is then modulated, preferably by the FSK method previously discussed.
Generator 8 is driven by the signal from generator 6 to produce the FSK modulated output which is alternately, at rate Fm~ of frequencies Fl and F2. Preferably, frequencies 1~49918 Fl and F2 deviate from the basic or central frequency Fc by the same amount, that is, by a frequency shift on the order e4r~d A f, for example, 25 Hz above the below, respectively, the central frequency. If the central frequency Fc is selected to be within the audio range, then the side band frequencies Fl and F2 are also audio frequencies.
m e output of generator 8 is passed through a band pass filter 10 and then, for safety, shaped, e. g., squared, and limited in the shaper and limiter unit 11 to provide a sub-stantially constant amplitude signal as the supply voltagefor the generator varies. This squared wave form is then applied to pre-amplifier 12 and power amplifier 13, which are coupled by interstage transformer 14, to furnish sufficient signal level for transmission through the rails, to which the amplifiers are coupled by output transformer 16. A final series LC band pas~ filter 17, tuned to the central frequency Fc~ iB inserted in the rail connections, to block interfering signals in the rails from the transmitter network. The trans-mitter arrangement thus applies to the rails 1 and 2 of the track circuit a signal characterized by having two side fre-quencies Fl and F2, which alternate in time at the modulation frequency Fm as driven by the modulation generator 6. This signal has substantially a constant amplitude, even though the supply voltage may vary, to permit a stability in the track circuit operation, that is, in the shunt condition which it will detect.

The track circuit receiver apparatus illustrated above the dashed divider line receives the track signal from the rails at the other end of section T through a band pass filter 19 which, for example, may be a series LC circuit tuned to a narrow frequency band, centered on the base fre-quency Fc but wide enough to include side frequencies Fl and F2. A second band pass filter 20 coupled by input transformer 21 is more selective than the initial filter 19, being tuned to pass the side frequencies Fl and F2. The output of filter 20 is coupled by the impedance amplifier unit 22 to a first and second verification circuit network outlined by the dashed blocks 23 and 24, respectively.
The first verification network includes a signal shaper 26, a demodulator 27, and a decoder element 28. Unit 26 shapes the signal received from filter 20 through the impe-dance amplifier into a square-wave form of constant amplitude and supplies this output to the demodulator 27. This latter unit demodulates the received signal only if the frequency deviation i6 identical to the predetermined shift in the transmitter apparatus, for example, + 25 Hz from the central carrier frequency Fc. If this condition exists, the output signal at the demodulation rate is applied to decoder unit 28, which inc1ude-q a resonant LC circuit. Unit 28 generates an output, that is, a concurrence or true signal, only if the frequency or code rate of the demodulated signal coincides with the modulation frequency or code rate Fm of the signal supplied by the transmitter apparatus. Thus, the first ~149918 verification network 23 produces a true or concurrence output signal only when the input from band pass filter 20 is an FSK signal, including frequencies Fl and F2, alternating at the modulation frequency Fm and based on, and with equal deviation from the central carrier frequency Fc as transmitted from the other end of the track circuit by the transmitter apparatus. In other words, a true signal output by network 23 designates that the received rail signal has frequency and dulation characteristics identical to the signal transmitted into the rails at the other end of the section.
The second verification circuit network 24 consists solely of a linear amplifier 32 with a preselected threshold level. Amplifier 32 produces an output signal only when the input received from band pass filter 20 through amplifier 22 equals or exceeds this threshold. This level is preselected in accordance with the calculated or known attenuation of the tran~mitted signal by the associated track circuit which, in this case, includes rails 1 and 2 of section T. Thus, an output signal from this amplifier signifies that the track signal received i8 from the transmitter apparatus at the other end because it has at least the expected minimum level.
The true or concurrence signal outputs of networks 23 and 24, specifically from decoder 28 and amplifier 32, re~pec-tively, are applied to the ~wo inputs of the logic D gate 29. As is conventional, element 29 produces an output signal only when both input signals are present. An output from element 29 is applied to a delay circuit 30 which has a ~1499~-8 predetermined delay time, for example, on the order of two to three seconds. Unit 30 produces sufficient energy to energize track relay TR but only when the input signal from AND gate 29 is of a longer duration than this predetermined delay time.
When energized, relay TR picks up, closing its front contact to indicate an unoccupied condition for section T. However, if relay TR becomes de-energized, for example, when the AND
element output ceases, it releases immediately to register an occupied track section condition. The delay time of unit 3~0 assures that relay TR does not pick up in response to input pulses of very short duration from an interference source which may, however, have frequency and amplitude character-istics equivalent to proper track signal. In other words, this delay time inhibits the response of relay TR to a variable noise source which might activate simultaneous re~ponse, for a very short interval, by verification networks 23 and 24.
The receiver apparatus, therefore, energizes track relay TR, to register an unoccupied track circuit condition, only when the signals received through the track circuit rails from the tran~mitter apparatus have the correct or true characteristics. These characteristics include being an FSK
modulated signal with the same side frequencies Fl and F2, deviating from a central carrier frequency Fc by the same frequency shift amount, having the same dulation frequency Fm as transmitted, an amplitude equal to or greater than a predetermined threshold level selected in accordance with the ~4~918 the known track circuit attenuation characteristics, and a ~-duration greater than a predetermined safety time period.
The satisfying of these conditions, without which track relay TR is not energized, renders the receiver apparatus very selective with regard to the signals received, thereby increasing its degree of nonresponse to interferin~ or noise signals present in the rails 1 and 2 of the track circuit.
Although I have herein shown and described but a single preferred form of a frequency modulated track circuit arrange-ment embodying the invention, it is to be understood thatvarious changes and modifications within the scope of the appended claims may be made without departing from the spirit and scope of my invention.

Claims (7)

Having now described the invention, what I claim as new and desire to secure by Letters Patent, is:
1. A frequency modulated track circuit for a section of railroad track, to detect the presence or absence of a train within that track section, comprising, (a) transmitter means coupled to the track rails at one end of said section and operable for transmitting a fre-quency modulated alternating current signal with first and second frequencies alternating at a pre-selected modulation rate, (b) a receiver means coupled to said rails at the other end of said section for receiving signals through said rails, and (c) a track relay normally occupying a first position and operable at times to a second position for indicating said track section occupied or unoccupied, respectively, by a train, (d) said receiver means further coupled to said track relay and responsive to received signals for opera-ting said relay to its second position only when the received signal includes said first and second fre-quencies alternating at said preselected modulation rate, is of at least a predetermined threshold level selected in accordance with rail circuit attenuation and known transmitted level, and maintains the modu-lation and level characteristics for greater than a preselected delay period.
2. A frequency modulated track circuit as defined in claim 1, in which, (a) said frequency modulated signal transmitted by said transmitter means comprises a central frequency signal, frequency shift keyed to produce said alternate first and second frequencies at said preselected modulation rate, and in which said receiver means includes, (b) a first verification network coupled to said rails and responsive to the frequency characteristics of the received track signal for producing an output signal only when the received signal consists of alternate first and second frequency signals shifted from said central frequency and alternating at said preselected modulation rate, (c) a second verification network coupled to said rails and responsive to the level of said received track signal for producing an output signal only when the level of said received signal is equal to or greater than a threshold level predetermined in accordance with the transmitted signal level and the attenua-tion characteristics of the track rails circuit path, and (d) a logic means coupled to said first and second veri-fication networks and responsive only to the recep-tion of both output signals for operating said track relay to its second position after said preselected delay period.
3. A track circuit as defined in claim 2 in which said receiver means further includes, (a) an input filter network coupling said first and second verification networks to said rails and tuned for passing from said rails to said networks only a received signal of said first and second frequencies shifted from said central frequency.
4. A track circuit arrangement as defined in claim 3, in which said first verification network comprises, (a) signal shaper means coupled to said input filter network for squaring the wave shape of the filter network output signal and producing a constant amplitude output, (b) a demodulator means coupled to said signal shaper means and responsive to an output signal therefrom for producing a code rate signal equivalent to the modulation rate of said first and second frequency signals received only if such signals are shifted from said central frequency, and (c) a decoding means coupled to receive the code rate signal from said demodulator means and responsive for supplying a signal to said logic means only if the demodulated code rate is identical with said preselected modulation rate.
5. A track circuit arrangement as defined in claim 4, in which said second verification network comprises, (a) a linear amplifier element having a threshold level predetermined in accordance with the level of the transmitted rail signal and the attenuation charac-teristics of said track rails, (b) said amplifier element responsive to a signal re-ceived from said input filter network for sup-plying an output signal to said logic means only when the received signal level is equal to or greater than said predetermined threshold level.
6. A track circuit arrangement as defined in claim 5 in which said logic means includes, (a) an AND gate coupled to said first and second veri-fication networks and responsive for producing an output signal only when output signals are re-ceived simultaneously from both verification net-works, and (b) a delay circuit means with said preselected delay time coupled between said AND gate and said track relay and responsive for operating said track re-lay to its second position only when said AND gate output signal has a duration greater than said preselected delay time.
7. A track circuit arrangement as defined in claim 6, in which said input filter network comprises, (a) a first band pass filter means coupled to the section rails and tuned to said central frequency for passing a band of frequencies including said first and second frequencies, (b) a second band pass filter means coupled to said first band pass filter means and tuned for passing said first and second frequencies, and (c) an amplifier means coupling said second band pass filter means to said first and second verification networks.
CA000342152A 1979-12-18 1979-12-18 Frequency modulated railroad track circuit Expired CA1149918A (en)

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Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1115213A1 (en) * 1998-09-03 2001-07-11 Hitachi, Ltd. Method and device for receiving frequency-modulated signal
US7017864B2 (en) 2001-11-21 2006-03-28 Westinghouse Brake And Signal Holdings Limited Railway track circuits
US8297558B2 (en) 2010-03-17 2012-10-30 Safetran Systems Corporation Crossing predictor with authorized track speed input
US8500071B2 (en) 2009-10-27 2013-08-06 Invensys Rail Corporation Method and apparatus for bi-directional downstream adjacent crossing signaling
US8590844B2 (en) 2009-07-17 2013-11-26 Siemens Rail Auotmation Corporation Track circuit communications
US8660215B2 (en) 2010-03-16 2014-02-25 Siemens Rail Automation Corporation Decoding algorithm for frequency shift key communications
CN104369746A (en) * 2014-08-29 2015-02-25 中铁二院工程集团有限责任公司 Signal processing method for phase-sensitive track circuit receiver

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1115213A1 (en) * 1998-09-03 2001-07-11 Hitachi, Ltd. Method and device for receiving frequency-modulated signal
EP1115213A4 (en) * 1998-09-03 2003-07-09 Hitachi Ltd Method and device for receiving frequency-modulated signal
US7017864B2 (en) 2001-11-21 2006-03-28 Westinghouse Brake And Signal Holdings Limited Railway track circuits
US8590844B2 (en) 2009-07-17 2013-11-26 Siemens Rail Auotmation Corporation Track circuit communications
US8500071B2 (en) 2009-10-27 2013-08-06 Invensys Rail Corporation Method and apparatus for bi-directional downstream adjacent crossing signaling
US9248849B2 (en) 2009-10-27 2016-02-02 Siemens Industry, Inc. Apparatus for bi-directional downstream adjacent crossing signaling
US8660215B2 (en) 2010-03-16 2014-02-25 Siemens Rail Automation Corporation Decoding algorithm for frequency shift key communications
US8297558B2 (en) 2010-03-17 2012-10-30 Safetran Systems Corporation Crossing predictor with authorized track speed input
CN104369746A (en) * 2014-08-29 2015-02-25 中铁二院工程集团有限责任公司 Signal processing method for phase-sensitive track circuit receiver

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